1. Field of the Invention
This invention relates to a photomask and a pattern transfer method and, in particular, to a photomask and a pattern transfer method for transferring, in a reduction projection exposure process, a minute pattern onto a wafer having a non-planar surface.
2. Description of the related art
Generally speaking, in a reduction projection exposure process, which is conducted to produce semiconductor devices etc., a photomask is used. The photomask includes a desired transfer pattern consisting of light transmissive and light shielding portions that are respectively transparent and opaque with respect to an illuminating light. FIG. 5 is a sectional view of a conventional photomask 1 of this type. The photomask 1 has a transparent substrate 2 in the form of a flat plate made of quartz or the like, and a metal layer 3 thereon in a desired pattern. Those portions where the metal layer 3 is present on the substrate 2 constitute the light shielding portions, and those portions consisting only of the transparent substrate 2 constitute the transmissive portions which transmit the illuminating light.
A pattern transfer method using this photomask 1 will be described with reference to FIG. 6. The illuminating light from a light source 4 is transmitted through a condenser lens 5 to illuminate the entire surface of the photomask 1. The illuminating light transmitted through the transmissive portions of the photomask 1 projects an image of the transfer pattern of the photomask 1 onto the surface of a wafer 7 through a projection lens system 6. A photoresist 9, serving as a photosensitive material, previously applied to the surface of a substrate 8 of the wafer 7 receives the image of the pattern of the photomask 1 on a surface 9a. By developing the photoresist 9, the pattern transfer is effected.
Assuming that the aberration in the projection lens system 6 is so small as to be negligible, the image formed on the surface 9a of the photoresist 9 of the wafer 7 by the illuminating light from the flat photomask 1 is planar. The image formation characteristics of the formed image are maintained within a range of a predetermined depth of focus D with respect to the direction of the optical axis of the illuminating light. That is, as shown in FIG. 6, satisfactory image formation characteristics can be obtained within the range between planes S1 between which the surface 9a of the photoresist 9 is located.
However, when semiconductor devices are produced, a large number of layers having various patterns are formed on a wafer substrate during circuit formation. As a result, in the reduction projection exposure process, the surface of the photoresist, onto which the pattern of the photomask is to be printed, cannot always be a uniform, flat surface but may be a non planar surface. For example, as shown in FIG. 7, the formation of an insulating layer 20 on a substrate 18 of a wafer 17 may result in a step difference A having a height H, between first and second ranges R1 and R2 having different surface levels.
When the height H of this difference A is within the range of the depth of focus D of the illuminating exposure apparatus, it is possible to obtain satisfactory image formation characteristics in both the first and second ranges R1 and R2 of the wafer 17. When, however, the difference A is in excess of the range of the depth of focus D, defocusing occurs in at least one of the ranges R1 and R2, resulting in a deterioration in the accuracy of the pattern transfer.
The depth of focus D varies in different illuminating exposure apparatus. A difference A having a height of 1 .mu.m or more is often incapable of being kept within the range of the depth of focus D, resulting in a deterioration in transfer accuracy.
A photomask for transferring a minute pattern onto a wafer having such a non-planar surface has been proposed in Japanese Patent publication 1-147458, and is shown in FIG. 8. As shown in the drawing, in the peripheral portions of first openings 11, which constitute the pattern to be transferred, are second, small openings 12 for generating a phase difference. Further, phase shift layers 13 are provided in the second openings 12. A light 15 coming from a light source and impinging upon the second openings 12 is changed in phase as it is transmitted through the phase shift layers 13 to interfere with a light 14 transmitted through the first openings 11. Due to this arrangement, an improvement is attained in the degree of process tolerances. However, this involves a change in the best focus position, depending upon the difference in phase between the light beams 14 and 15. In view of this, the phase shift layers 13, in the second openings 12, are designed to produce a phase difference corresponding to the height of the wafer onto which an image of the openings 11 is to be projected. By using this photomask, it is possible to transfer a minute pattern onto the entire area of a wafer having a difference in surface level of approximately .+-.1 .mu.m.
A problem with this photomask is that the second, minute openings 12 for phase difference generation have to be located in the peripheral portions of the first openings 11 to be transferred, with the result that the pattern formed on the photomask is very complicated. Further, the difference in phase, which has an influence on the best focus position within the range of 0.degree. to 360.degree., remains the same even in the range beyond 360.degree. because of its cyclical nature. Thus, although this photomask utilizing phase difference, disclosed in the above publication, is effective with respect to a wafer having a difference in surface level of approximately .+-.1 .mu.m or less, it is difficult for the photomask to attain the best focus over the entire area with respect to a wafer having a larger difference in surface level.